Fast axis beam profile shaping by collimation lenslets for high power laser diode based annealing system

ABSTRACT

A dynamic surface anneal apparatus for annealing a semiconductor workpiece has a workpiece support for supporting a workpiece, an optical source and scanning apparatus for scanning the optical source and the workpiece support relative to one another along a fast axis. The optical source includes an array of laser emitters arranged generally in successive rows of the emitters, the rows being transverse to the fast axis. Plural collimating lenslets overlie respective ones of the rows of emitters and provide collimation along the fast axis. The selected lenslets have one or a succession of optical deflection angles corresponding to beam deflections along the fast axis for respective rows of emitters. Optics focus light from the array of laser emitters onto a surface of the workpiece to form a succession of line beams transverse to the fast axis spaced along the fast axis in accordance with the succession of deflection angles.

FIELD OF THE INVENTION

The invention relates generally to thermal processing of semiconductorsubstrates. In particular, the invention relates to laser thermalprocessing of semiconductor substrates.

BACKGROUND ART

Thermal processing is required in the fabrication of silicon and othersemiconductor integrated circuits formed in silicon wafers or othersubstrates such as glass panels for displays. The required temperaturesmay range from relatively low temperatures of less than 250 degrees C.to greater than 1400 degrees C. and may be used for a variety ofprocesses such as dopant implant annealing, crystallization, oxidation,nitridation, silicidation, and chemical vapor deposition as well asothers.

For the very shallow circuit features required for advanced integratedcircuits, it is greatly desired to reduce the total thermal budget inachieving the required thermal processing. The thermal budget may beconsidered as the total time at high temperatures necessary to completedevice fabrication. The time that the wafer needs to stay at the highesttemperature can be very short. The greater the total time that the waferis subject to high temperatures, the more features such as implantedjunctions will diffuse and loose their definition. For example,implanted junction depths may become deeper than desired due todiffusion.

Rapid thermal processing (RTP) uses radiant lamps which can be veryquickly turned on and off to heat only the wafer and not the rest of thechamber. Pulsed laser annealing using very short (about 20 ns) laserpulses is effective at heating only the surface layer and not theunderlying wafer, thus allowing very short ramp up and ramp down rates.

A more recently developed approach in various forms, sometimes calledthermal flux laser annealing or dynamic surface annealing (DSA), isdescribed in U.S. Pat. Nos. 7,005,601 and 6,987,240, the disclosures ofwhich are incorporated herein by reference. The DSA system employs manyCW diode lasers focused on an extremely narrow (0.07 mm) line beam toproduce a very intense beam of light that strikes the wafer as a thinlong line of radiation or line beam. The line beam is then scanned overthe surface of the wafer in a direction perpendicular to the longdimension of the line beam.

The thinness of the line beam (e.g., 0.07 mm) ensures very shorttemperature rise and fall times and a short dwell time at the requiredtemperature, e.g., 1300 degrees C., with respect to a fixed location onthe wafer surface that is scanned once by the line beam. For example,the temperature of a fixed location on the wafer surface will increasefrom an ambient 450 degrees C. to 1300 degrees C. within 0.6 ms,assuming a scan rate within a range of about 50-300 mm/sec is employed.The advantage is that the wafer surface spends an extremely short amountof time at lower or intermediate temperatures (e.g., 500-900 degrees C.)at which the higher silicon thermal conductivity promotes heatingthroughout the wafer and consequent diffusion and loss of underlyingcircuit feature definition. Instead, the wafer surface spends more timeat the desired high temperature (e.g., 1300 degrees c.) at which siliconthermal conductivity is lowest for minimum heating of the underlyingfeatures, and at which desired effects are maximum (e.g., annealing ofimplanted dopant impurities, annealing of pre-implant amorphizationdamage, etc.). The thinness of the line beam corresponds to the minimumresolvable spot size R of the laser beam optical system, which isgoverned by the following approximate formula:R=λ/2 NA,where λ is the laser wavelength and NA is the numerical aperture of theoptics. Numerical aperture is defined as:NA=n sin θ/2,where n is the index of refraction and θ is the angle subtended by thebeam between the aperture at the lens and the focal point in a simple orideal system. In the DSA system referred to above, the wavelength is 810nm and the angle θ is less than 60 degrees and n is the index ofrefraction of air (about 1).

These parameters provide a minimum resolvable spot size R correspondingto the small width of the line beam (0.07 mm). Within the preferred beamscanning rate range (50-300 mm/sec), each fixed wafer surface locationspends less than about 0.5 ms near the peak temperature (e.g., 1300degrees C.).

The required level of the wafer surface peak temperature (1300 degreesC.) requires a power density of about 220 kiloWatts/cm². To reach thislevel, the DSA system employs a large number of 810 nm CW diode lasersfocused on the same line beam image, as will be described later in thisspecification.

One problem recently encountered is that some annealing processesrequires a longer time at or near the peak temperature (1300 degreesC.), than the current dwell time of less than 0.5 ms. This dwell timemay be sufficient to cause ion implanted dopant impurities to becomesubstitutional in the semiconductor crystal lattice. However, it may beinsufficient to completely cure pre-ion implantation amorphizationdefects. Pre-ion implantation amorphization is performed prior to ionimplantation of dopant impurities to form shallow PN junctions toprevent channeling of the kinetic dopant ions through the crystallattice below the desired junction depth. Amorphization prevents suchchanneling by ion bombardment of the wafer with heavier atomic species(oxygen, nitrogen, carbon), causing bombardment damage to at leastpartially convert the active semiconductor layer from a crystallinestate to an amorphous state. The defects in the crystal are curedprovided each wafer surface location has a sufficiently long dwell timenear 1300 degrees C. This may require a dwell time that is longer thanthe current 0.5 ms dwell time. Furthermore, conversion of the amorphizedregion back to a crystalline state essentially forms an epitaxialcrystalline layer over the bulk crystalline layer, giving rise toanother class of defects, namely boundary defects at the interfacebetween the bulk crystal and the re-crystallized surface.

Such boundary defects have been found to be more persistent than theother types of defects, and require a significantly longer dwell time tocompletely cure or remove, as long as 2 to 3 ms near 1300 degrees C.

In order to provide such a long dwell time, the beam spot size must beenlarged, which essentially broadens the Gaussian profile of the beamintensity along the direction of scan, hereinafter referred to as the“fast axis”. Unfortunately, if the Gaussian beam profile is widened by agiven factor, then the slope of the leading edge of the Gaussian beamprofile is reduced by approximately the same factor. This increases thetemperature rise time (and fall time), thereby subjecting each locationon the wafer surface to a longer time at a lower or intermediatetemperature, and thus degrading the device structure through thermaldiffusion, for example. Thus, widening the beam profile along the fastaxis does not appear to be possible. Therefore there is a need for a DSAprocess in which the boundary defects and other defects in an ionimplanted wafer can be annealed without device degradation due togreater thermal diffusion.

SUMMARY OF THE INVENTION

A dynamic surface anneal apparatus for annealing a semiconductorworkpiece has a workpiece support for supporting a workpiece, an opticalsource and scanning apparatus for scanning the optical source and theworkpiece support relative to one another along a fast axis. The opticalsource includes an array of laser emitters arranged generally insuccessive rows of the emitters, the rows being transverse to the fastaxis. Plural collimating lenslets overlie respective ones of the rows ofemitters and provide collimation along the fast axis. The selectedlenslets have one or a succession of optical deflection anglescorresponding to beam deflections along the fast axis for respectiverows of emitters. Optics focus light from the array of laser emittersonto a surface of the workpiece to form a succession of line beamstransverse to the fast axis spaced along the fast axis in accordancewith the succession of deflection angles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an orthographic representation of a thermal flux laserannealing apparatus employed in the present invention.

FIGS. 2 and 3 are orthographic views from different perspectives ofoptical components of the apparatus of FIG. 1.

FIG. 4 is an end plan view of a portion of a semiconductor laser arrayin the apparatus of FIG. 1.

FIG. 5 is an orthographic view of a homogenizing light pipe for theapparatus of FIG. 1.

FIG. 6 is a schematic diagram corresponding to FIGS. 2 and 3.

FIG. 7 is a perspective view of a portion of the beam source of anembodiment of the invention.

FIG. 8 is an end view corresponding to FIG. 7 and further illustrating acontroller for varying the fast axis power density profile of the twoline beams.

FIG. 9 is a graph depicting the fast-axis power density profile obtainedwith the apparatus of FIGS. 7 and 8.

FIG. 10 is a schematic diagram of another embodiment of the inventionemploying several displace Gaussian beams with a programmable fast-axisprofile.

FIGS. 11A through 11E are graphs of different fast-axis power densityprofiles that can be selected with a programmable controller of the DSAapparatus of FIG. 10.

FIG. 12A is a graph depicting a preferred power density profile overtime generated with the DSA apparatus of FIG. 10.

FIG. 12B is a graph contemporaneous with the graph of FIG. 12A anddepicting the time behavior of the temperature of a fixed spot on thewafer surface resulting from the power density profile of FIG. 12A.

FIG. 13 is a cut-away side view of a semiconductor device formed usingthe DSA apparatus of the invention.

FIG. 14 depicts a DSA process employing the apparatus of FIG. 10.

FIG. 15 is a schematic diagram depicting a first modification of theembodiment of FIG. 8 employing beam deflecting mirrors.

FIG. 16 is a schematic diagram depicting a second modification of theembodiment of FIG. 8 employing a single beam deflecting mirror.

FIGS. 17 and 18 depict a third modification of the embodiment of FIG. 8in which selected ones of the fast-axis collimating cylindrical lensesare rotated through the desired beam-deflecting angle.

FIG. 19 is a schematic diagram depicting a first modification of theembodiment of FIG. 10 in which the prisms of successively greater beamdeflection angles are replaced by rotating the corresponding fast-axiscylindrical lenses through successively greater angles. FIG. 20 is aschematic diagram depicting a second modification of the embodiment ofFIG. 10 in which the prisms of successively greater beam deflectionangles are replaced by respective beam deflecting mirrors rotatedthrough successively greater angles.

DETAILED DESCRIPTION OF THE INVENTION

Introduction:

Defects in an ion implanted wafer of the type that persist beyond a halfmillisecond at high temperature (1300 degrees C.) are annealed or curedby providing a very long dwell time (e.g., 2-3 milliseconds) withoutcompromising the extremely steep rising and falling edges of theGaussian shaped line beam along the fast axis. In this way, boundarydefects that are an artifact of pre-implant amorphization and postimplant annealing are completely removed without incurring acorresponding penalty in thermal diffusion. All this is accomplished byfocusing a first set of lasers on a first line beam image and focusing asecond set of lasers on a second line beam image whose amplitude peak isdisplaced from the amplitude peak of the first line beam image along thedirection of the fast axis (i.e., perpendicular to the length of theline beam). This displacement is preferably the width of the Gaussianprofile of the line beam along the fast axis at an amplitudecorresponding to half the peak amplitude of one of the two line beams.Both line beams have the same highly focused image with the minimumresolvable spot size of about 0.07 mm as before. The angle subtendedbetween the optical paths of the two sets of lasers is less than onedegree in order to achieve such a small displacement, and this angledepends upon the distance between the lasers and the wafer surface.

The net effect is the same extremely steep slope of the leading andtrailing beam edges along the fast axis as with a single line beam, butwith a dwell time at or near the peak temperature that is doubled fromthat of a single line beam. One of the laser line beams is the leadingbeam while the other is the trailing beam. The leading beam must raisethe wafer surface temperature from about 400 degrees C. to 1300 degreesC. within 0.5 ms, and must therefore be of higher power density, whilethe trailing beam must simply maintain the wafer surface at the elevatedtemperature (without increasing the temperature), and is therefore of alesser power density.

The time-profile of the wafer surface temperature at each fixed locationmay be adjusted by adjusting the currents supplied to the two sets oflasers. Finer adjustment may be realized by providing a larger number oflaser sets focused respectively on the corresponding number of linebeams (e.g., four line beams), and programming the four current levelssupplied to the four laser sets.

DSA Apparatus:

One embodiment of the DSA apparatus described in the above-referencedpatent application is illustrated in the schematic orthographicrepresentation of FIG. 1. A gantry structure 10 for two-dimensionalscanning includes a pair of fixed parallel rails 12, 14. Two parallelgantry beams 16, 18 are fixed together a set distance apart andsupported on the fixed rails 12, 14 and are controlled by anunillustrated motor and drive mechanism to slide on rollers or ballbearings together along the fixed rails 12, 14. A beam source 20 isslidably supported on the gantry beams 16, 18, and may be suspendedbelow the beams 16, 18 which are controlled by unillustrated motors anddrive mechanisms to slide along them. A silicon wafer 22 or othersubstrate is stationarily supported below the gantry structure 10. Thebeam source 20 includes a laser light source and optics (describedbelow) to produce two closely spaced downwardly directed fan-shapedbeams 24 and 25 that strike the wafer 22 as leading and trailing linebeams 26 and 27, respectively, extending generally parallel to the fixedrails 12, 14, in what is conveniently called the slow direction. As willbe described below, the second line beam 27 is parallel to the firstline beam 26 and displaced from it by a distance corresponding to theminimum resolvable spot size of the optical system. How that isaccomplished is discussed later in this specification.

Although not illustrated here, the gantry structure further includes aZ-axis stage for moving the laser light source and optics in a directiongenerally parallel to the fan-shaped beam 24 to thereby controllablyvary the distance between the beam source 20 and the wafer 22 and thuscontrol the focusing of the line beam 26 on the wafer 22. Exemplarydimensions of the line beam 26 include a length of 1 cm and a width of66 microns with an exemplary power density of 220 kW/cm². Alternatively,the beam source and associated optics may be stationary while the waferis supported on a stage which scans it in two dimensions.

In typical operation, the gantry beams 16, 18 are set at a particularposition along the fixed rails 12, 14 and the beam source 20 is moved ata uniform speed along the gantry beams 16, 18 to scan the line beams 26,27 perpendicularly to its long dimension in a direction convenientlycalled the fast direction. The two line beams 26, 27 are thereby scannedfrom one side of the wafer 22 to the other to irradiate a 1 cm swath ofthe wafer 22. The line beams 26, 27 are sufficiently narrow and thescanning speed in the fast direction is sufficiently fast so that aparticular area of the wafer is only momentarily exposed to the opticalradiation of the line beams 26, 27 but the intensity at the peak of theline beam is sufficient to heat the surface region to very hightemperatures. However, the deeper portions of the wafer 22 are notsignificantly heated and further act as a heat sink to quickly cool thesurface region. Once the fast scan has been completed, the gantry beams16, 18 are moved along the fixed rails 12, 14 to a new position suchthat the line beam 26 is moved along its long dimension extending alongthe slow axis. The fast scanning is then performed to irradiate aneighboring swath of the wafer 22. The alternating fast and slowscanning are repeated, perhaps in a serpentine path of the beam source20, until the entire wafer 22 has been thermally processed.

The optics beam source 20 includes an array of lasers in order torealize the high optical power density (220 kW/cm²) required. Theoptical system described below focuses the beams from the array oflasers into two closely spaced parallel line beams, each of width 66microns. One optical system that is suitable for doing this isorthographically illustrated in FIGS. 2 and 3, in which laser radiationat about 810 nm is produced in an optical system 30 from two laser barstacks 32, one of which is illustrated in end plan view in FIG. 4. Eachlaser bar stack 32 includes 14 parallel bars 34, generally correspondingto a vertical p-n junction in a GaAs semiconductor structure, extendinglaterally about 1 cm and separated by about 0.9 mm. Typically, watercooling layers are disposed between the bars 34. In each bar 34 areformed 49 emitters 36, each constituting a separate GaAs laser emittingrespective beams having different divergence angles in orthogonaldirections. The illustrated bars 34 are positioned with their longdimension extending over multiple emitters 36 and aligned along the slowaxis and their short dimension corresponding to the less than 1-micronp-n depletion layer aligned along the fast axis. The small source sizealong the fast axis allows effective collimation along the fast axis.The divergence angle is large along the fast axis and relatively smallalong the slow axis.

Returning to FIGS. 2 and 3, individual half-cylindrical lenslets 40 arepositioned over individual laser bars 34 to collimate the laser light ina narrow beam along the fast axis. The lensets 40 may be bonded withadhesive on the laser stacks 32 and aligned with the bars 34 to extendover the emitting areas 36. As will be described in more detail later inthis specification, some of the laser bars 34 and their lenlets 40 arecovered by prisms 44 that deflect the light by an angle less than 1degree to form the second (trailing) line beam 27 depicted in FIG. 1.The angle of the prisms 44 and the displacement between the two linebeams 26, 27 are so infinitesimally small relative to the scale of thedrawings of FIGS. 1-4 that their size in the drawings has been greatlyexaggerated in order for them to be slightly visible in the drawings.

The optics beam source 20 can further include conventional opticalelements. Such conventional optical elements can include an interleaverand a polarization multiplexer, although the selection by the skilledworker of such elements is not limited to such an example. In theexample of FIGS. 2 and 3, the two sets of beams from the two bar stacks32 are input to an interleaver 42, which has a multiple beam splittertype of structure and having specified coatings on two internal diagonalfaces, e.g., reflective parallel bands, to selectively reflect andtransmit light. Such interleavers are commercially available fromResearch Electro Optics (REO). In the interleaver 42, patterned metallicreflector bands are formed in angled surfaces for each set of beams fromthe two bar stacks 32 such that beams from bars 34 on one side of thestack 32 are alternatively reflected or transmitted and therebyinterleaved with beams from bars 34 on the other side of the stack 32which undergo corresponding selective transmission/reflection, therebyfilling in the otherwise spaced radiation profile from the separatedemitters 36.

A first set of interleaved beams is passed through a half-wave plate 48to rotate its polarization relative to that of the second set ofinterleaved beams. Both sets of interleaved beams are input to apolarization multiplexer (PMUX) 52 having a structure of a doublepolarization beam splitter. Such a PMUX is commercially available fromCVI Laser Inc. First and second diagonal interface layers 54, 56 causethe two sets of interleaved beams to be reflected along a common axisfrom their front faces. The first interface 54 is typically implementedas a dielectric interference filter designed as a hard reflector (HR)while the second interface 56 is implemented as a dielectricinterference filter designed as a polarization beam splitter (PBS) atthe laser wavelength. As a result, the first set of interleaved beamsreflected from the first interface layer 54 strikes the back of thesecond interface layer 56. Because of the polarization rotationintroduced by the half-wave plate 48, the first set of interleaved beamspasses through the second interface layer 56. The intensity of a sourcebeam 58 output by the PMUX 52 is doubled from that of the either of thetwo sets of interleaved beams.

Although shown separated in the drawings, the interleaver 42, thehalf-wave plate 48, and the PMUX 52 and its interfaces 54, 56, as wellas additional filters that may be attached to input and output faces aretypically joined together by a plastic encapsulant, such as a UV curableepoxy, to provide a rigid optical system. There is a plastic bondbetween the lenslets 40 and the laser stacks 32, on which they arealigned to the bars 34. The source beam 58 is passed through a set ofcylindrical lenses 62, 64, 66 to focus the source beam 58 along the slowaxis.

A one-dimensional light pipe 70 homogenizes the source beam along theslow axis. The source beam, focused by the cylindrical lenses 62, 64,66, enters the light pipe 70 with a finite convergence angle along theslow axis but substantially collimated along the fast axis. The lightpipe 70, more clearly illustrated in the orthographic view of FIG. 5,acts as a beam homogenizer to reduce the beam structure along the slowaxis introduced by the multiple emitters 36 in the bar stack 32 spacedapart on the slow axis. The light pipe 70 may be implemented as arectangular slab 72 of optical glass having a sufficiently high index ofrefraction to produce total internal reflection. It has a shortdimension along the slow axis and a longer dimension along the fastaxis. The slab 72 extends a substantial distance along an axis 74 of thesource beam 58 converging along the slow axis on an input face 76. Thesource beam 58 is internally reflected several times from the top andbottom surfaces of the slab 72, thereby removing much of the texturingalong the slow axis and homogenizing the beam along the slow axis whenit exits on an output face 78. The source beam 58, however, is alreadywell collimated along the fast axis (by the cylindrical lenslets 40) andthe slab 72 is wide enough that the source beam 58 is not internallyreflected on the side surfaces of the slab 72 but maintains itscollimation along the fast axis.

The light pipe 70 may be tapered along its axial direction to controlthe entrance and exit apertures and beam convergence and divergence. Theone-dimensional light pipe can alternatively be implemented as twoparallel reflective surfaces corresponding generally to the upper andlower faces of the slab 72 with the source beam passing between them.The source beam output by the light pipe 70 is generally uniform.

As further illustrated in the schematic view of FIG. 6, furtheranamorphic lens set or optics 80, 82 expands the output beam in the slowaxis and includes a generally spherical lens to project the desired linebeam 26 on the wafer 22. The anamorphic optics 80, 82 shape the sourcebeam in two dimensions to produce a narrow line beam of limited length.In the direction of the fast axis, the output optics have an infiniteconjugate for the source at the output of the light pipe (althoughsystems may be designed with a finite source conjugate) and a finiteconjugate at the image plane of the wafer 22 while, in the direction ofthe slow axis, the output optics has a finite conjugate at the source atthe output of the light pipe 70 and a finite conjugate at the imageplane. Further, in the direction of the slow axis, the nonuniformradiation from the multiple laser diodes of the laser bars ishomogenized by the light pipe 70. The ability of the light pipe 70 tohomogenize strongly depends on the number of times the light isreflected traversing the light pipe 70. This number is determined by thelength of the light pipe 70, the direction of the taper if any, the sizeof the entrance and exit apertures as well as the launch angle into thelight pipe 70. Further anamorphic optics focus the source beam into theline beam of desired dimensions on the surface of the wafer 22.

Programmable Beam Profile for Dwell Time and Temperature ProfileControl:

Referring to FIGS. 6 and 8, one group of laser bars 34 (e.g., alternatebars 34) radiate through corresponding cylindrical lenslets 40 to theoptics 42, 52, etc. The remaining ones of the lasers bars 34 radiatethrough corresponding cylindrical lenslets 40 and through individualprisms 44 overlying respective ones of the lenslets 40. Each prism 44deflects the beam by an angle (A) less than 1 degree through an angle ofrotation about an axis parallel with the fast axis to produce a beamdeflection perpendicular to the fast axis. The first group of laser bars34 whose beams are undeflected generate the leading beam 24. The secondgroup of laser bars 34 whose beams are deflected by the respectiveprisms 44 generate the trailing beam 25. The perspective view of FIG. 7shows two of the laser bars 34 forming two parallel rows of emitters 36,both covered by respective cylindrical lenslets 40, and one of thelenslets 40 being covered by a prism 44. FIG. 8 illustrates how thepower densities of the leading and trailing beams 24, 25 areindependently governed by a profile controller 100 controlling currentsupplies 102, 104 that drive the emitters 36 of alternate laser bars 34.The first current supply 102 is the front beam current supply because itis coupled to each of the laser bars not covered by any of the prisms40. The second current supply 104 is the trailing beam current supplybecause it is coupled to each of the laser bars covered by a prism 44.Each prism 44 deflects the corresponding laser beam to form the trailingbeam 25. The two line beams 26, 27 (FIG. 1) imaged on the wafer 22 bythe leading and trailing beams 24, 25 are separated by a displacementdetermined by the deflection angle of the prisms 44. The terms “front”and “trailing” apply to an embodiment in which the beam is scannedacross the wafer in a particular direction. If this direction isreversed, then the beams deflected by the prisms 44 form the leading or“front” beam 26 while the undeflected beams form the trailing beam 27.The invention may be carried out in either mode, and therefore the termssuch as “front” and “trailing”, for example, are employedinterchangeably with respect to the embodiment of FIG. 6.

In operation, the power density of the trailing line beam 27 may besignificantly less than that of the front line beam 26. This is becausethe front line beam 26 must have sufficient power density to raise thetemperature of the wafer surface rapidly through the lower temperatureranges where the higher thermal conductivity of the wafer makes it moredifficult to heat the surface, until the wafer surface reaches the peaktemperature. The trailing line beam merely maintains this temperature,which requires less power density, to avoid raising the wafer surfacetemperature beyond the maximum desired temperature (1300 degrees C.).Therefore, as indicated in FIG. 9, the trailing line beam has a fastaxis profile whose peak power density is significantly lower than thatof the leading line beam. The difference between the peak power densitylevels of the two beams is set by the profile controller 100 of FIG. 8.

FIG. 10 depicts an embodiment providing several (i.e., four) trailingbeams in succession, each one with an independently adjustable powerdensity. In FIG. 10, the laser bars 34 are divided into four groups. Theone group of laser bars 34-0 produce laser radiation that isundeflected, to form the front beam. Another group of laser bars 34-1produce radiation that is deflected by prisms 44-1 through a small angleA1 to produce the first trailing beam. A further group of laser bars34-2 produce radiation that is deflected by prisms 44-2 through an angleA2 that is greater than A1. Another group of laser bars 34-3 produceradiation that is deflected by prisms 44-3 through an angle A3 that isgreater than A2. The four line beams 26-1, 26-2, 26-3, 26-4 (FIG. 11A)are focused by the optics (42, 52, etc. of FIGS. 2-6) on the wafersurface. In the special case in which the four line beams 26-1, 26-2,26-3, 26-4 have the same power density, their power density profilesalong the fast axis are of identical Gaussian shapes, but are shiftedfrom one another along the fast axis by the same peak-to-peakdisplacement, as depicted in FIG. 11A. This displacement is determinedby the succession of deflection angles A1, A2, A3 imposed by the prisms44-1, 44-2, 44-3. Preferably, the angles A1, A2, A3 are selected so thatthe peak-to-peak displacement between neighboring line beams is at leastapproximately (if not exactly) equal to the half-maximum beam width(depicted in FIG. 9) of a single line beam (e.g., of the front beam).Equivalently, the displacement may correspond to the minimum resolvablespot size of the beam, discussed previously in this specification. Thedeflection angles are all less than 1 degree and in reality would not bedetectable in the drawing of FIG. 10. These angles have been exaggeratedin FIG. 10 for the sake of illustration.

The power density levels produced by the different groups of laser bars34-0, 34-1, 34-2, 34-3 are independently adjustable by a laser powercontroller 110 that furnishes independent supply currents I0, I1, I2, I3to the respective laser bars 34-0, 34-1, 34-2, 34-3. The laser powercontroller 110 therefore controls the fast axis power density profileproduced by the array of lasers. While their power densities may be thesame (corresponding to the power density profile of FIG. 11A), it ispreferable for the power densities of the trailing line beams to be lessthan that of the front or leading line beam, in accordance with thevarious power density profiles of FIG. 11B, 11C or 11D. The successiveline beam supply current levels I0, I1, I2, I3 may be adjusted by thecontroller 110 to produce a staircase profile of successively decreasingpower density levels as in FIG. 11B, or the single staircase of FIG.11C, or the gradual staircase of FIG. 11D. As another alternative, theamplitude profile may fall after the front or leading line beam and thenincrease from the third to last line beam, as depicted in FIG. 11E.Although not depicted in the drawings, the profile may be furtheradjusted to produce an ascending staircase pattern instead of thedescending staircase profiles of FIGS. 11B-11D.

The presently preferred power density profile of the succession of linebeams is illustrated in FIG. 12A. FIG. 12A depicts the power densityincident upon a particular spot on the wafer surface as a function oftime. (If FIG. 12A were converted to depict power density distributionalong the fast axis at a fixed instant in time, then the graph wouldremain unchanged, depending upon the units chosen.) The leading linebeam 26-1 has the highest peak power density, while the successivetrailing line beams 26-2, 26-3, 26-4 have a lesser peak power density,which is the same for each of them. The strong leading beam 26-1 of FIG.12A has sufficient power density to overcome the high silicon thermalconductivity to rapidly heat the wafer surface spot to 1300 degrees C.The trailing beams 26-2, 26-3, 26-4 that follow have a lesser powerdensity that is sufficient only to maintain the 1300 degree C. wafertemperature at the spot and not exceed it. FIG. 12B depicts thetemperature behavior over time of the same wafer surface spot thatresults from the beam profile of FIG. 12A. The temperature rises rapidlyfrom 400 degrees C. to 1300 degrees C. with the leading edge of thefront beam. The temperature of the spot then remains at about 1300degrees C. for 3 ms, with slight undulations in the temperaturecorresponding with the peaks of the successive beams. After about 3 ms,the temperature falls rapidly to 400 degrees C. with the trailing edgeof the last line beam.

FIG. 12B indicates the succession of effects of the laser radiation in apost-ion implantation DSA annealing process using the beam profile ofFIG. 12A. During an initial time interval (T1), ion bombardment damagein the semiconductor material incurred during a pre-implantamorphization process is annealed to convert the semiconductor materialfrom a partially amorphous state to a crystalline state. During afurther time interval, T2, the implanted dopant impurities are renderedsubstitutional in the semiconductor crystal lattice. During a yetfurther time interval T3, boundary defects formed at the interfacebetween the bulk crystal and the re-crystallized zone are cured. FIG. 13illustrates the location of such boundary defects. The boundary defectsarise when the amorphized semiconductor surface layer is re-crystallizedto form an epitaxial crystal layer over the bulk crystal. The twocrystal zones may not align perfectly at the boundary between them,giving rise to misalignments that are defects. Such defects may requireas long as 3 ms at 1300 degrees C. (i.e., the entire duration of themultiple line beams of FIG. 12A) to completely anneal or cure.

FIG. 14 depicts an ion implantation process for forming ultra-shallow PNjunctions such as source-drain extension implants in the surface of asilicon wafer. The structure to be formed is depicted in FIG. 13, inwhich ion implanted source-drain extensions 200, 205 are implantedbetween deep source-drain contact regions 210, 215 and a semiconductorchannel region 220 underlying a gate electrode 225 insulated from thechannel region 220 by a thin gate dielectric layer 230. In the firststep of the process of FIG. 14 (block 250), the surface region extendingfrom the top surface 235 of the wafer to the horizontal dashed line 240is converted from a pure crystalline state to an at least partiallyamorphous state by ion bombarding the wafer with heavy ions (e.g.,oxygen, nitrogen, carbon, germanium) to break the crystal bonds. Thision bombardment is carried out at an energy level at which thedistribution of ions in the wafer extends down to the dashed line 240and is virtually cut-off below that line. The purpose of thisamorphization step is to prevent channeling of dopant impurity ions thatare to be implanted in the next step of the process. Channeling is madepossible by the regular crystalline structure of the silicon wafer, andis prevented by converting the crystalline structure to an amorphous oneto a sufficient depth (corresponding to the dashed line 240 of FIG. 13).

The next step (block 255 of FIG. 14) is to implant a dopant impurity toform, for example, the source drain extensions 210, 215. The dopantimpurity may be As, P, B or other species. This step may be preceded bymasking steps to shield areas of the wafer that are not to be implantedin this step. The ion energy is selected so that the implanted iondistribution does not extend below the desired depth, such as theultra-shallow depth of the source drain extensions 210, 215 for example.

Optionally, prior to performing a DSA process employing the apparatus ofFIGS. 1-10, an optical absorber layer may be deposited on the wafersurface (block 260 of FIG. 14). This step may be carried out inaccordance with the low-temperature plasma process and apparatusdescribed in U.S. patent application Ser. No. 11/131,904, filed May 17,2005, entitled A SEMICONDUCTOR JUNCTION FORMATION PROCESS INCLUDING LOWTEMPERATURE PLASMA DEPOSITION OF AN OPTICAL ABSORPTION LAYER ND HIGHSPEED OPTICAL ANNEALING by Kartik Ramaswamy, et al. and assigned to thepresent assignee. The optical absorber layer may be amorphous carbon,for example.

The next step (block 270) is to perform the scanning laser DSA processusing multiple line beams with a configurable beam profile. A firstsub-step (block 271) of this step is to rapidly raise the temperature ofa newly encountered wafer surface spot (or line of spots) from anambient temperature of 400-450 degrees C. up to 1300 degrees C. usingthe steep leading edge of the front beam 26-1 (FIG. 12A). The nextsub-step (block 272) is to maintain the wafer temperature at about 1300degrees C. for a sufficient time (e.g., 3 ms) to (a) re-crystallize theamorphized surface region, (b) render the implanted dopant impuritiessubsitutional in the re-crystallized lattice and (c) cure the defects atthe boundary between the re-crystallized zone and the underlying bulkcrystal. The final sub-step (block 273) is to rapidly reduce the spotsurface temperature in accordance using the steep trailing edge of thelast trailing beam 26-4 (FIG. 12A).

FIG. 15 depicts a modification of the embodiment of FIG. 8 in which theprisms 44 are replaced by separate mirrors 121, 122, 123 that deflectthe beams from alternate laser bars 34 in the same manner that theprisms 44 deflected the beams in the embodiment of FIG. 8. If the beamdeflection angle is A, then the angle of each mirror 121, 122, 123relative to the beam direction emerging from each laser bar 34 is A/2.

FIG. 16 depicts a simpler version of the embodiment of FIG. 15, in whicha single optical element 120, which may be a mirror or a prism, deflectsthe beams from a succession of lasers bars 34-1, 34-2, 34-3, etc., toproduce the trailing line beam, while the beams from the other laserbars 34-4, 34-5, 34-6 are undeflected to form the leading line beam. Inthe case in which the optical element 120 of FIG. 16 is a mirror ratherthan a prism, for a beam deflection angle A, the angle of the opticalelement 120 relative to the beam direction emerging from each laser bar34 is A/2, as in the embodiment of FIG. 15.

FIGS. 17 and 18 depict a modification of the embodiment of FIGS. 7 and8, in which the prisms 44 are eliminated and, instead, different (i.e.,alternate) ones of the half-cylindrical lenses 40 are rotated throughthe angle A. Rotating the selected cylindrical lenses 40 produces thesame beam deflection as did the prisms 44 of FIGS. 7 and 8. Thehalf-cylindrical lenses 40 are mounted on the respective laser bars 34and aligned to produce the desired beam direction and then bonded to thelaser bars, preferably with UV curable epoxy. Thus, half the lenses(40-1, 40-3, 40-5) are aligned to provide a beam direction deflected bythe angle A, while the remaining lenses 40-2, 40-4, 40-6 are aligned toprovide an undeflected beam direction.

FIG. 19 depicts a modification of the embodiment of FIG. 10 in which theprisms 44-1, 44-2, 44-3 of successively greater beam angles areeliminated and their beam deflection functions are provided instead byrotating corresponding ones of the half-cylindrical lenses (40-1, 40-2,40-3) through successively greater angles A1, A2, A3. The lens 40-0remains unrotated, to provide four successive beam angles of 0, A1, A2and A3. This is identical to the succession of beam angles provided tothe optics 42, 52, in the embodiment of FIG. 10.

FIG. 20 depicts a modification of the embodiment of FIG. 10 in which theprisms 44-1, 44-2, 44-3 of successively greater beam angles are replacedby mirrors 120-1, 120-2, 120-3 rotated by successively greater anglesA₁/2, A₂/2, A₃/2 deflecting the beams from the laser bars 34-1, 34-2,34-3, respectively. The beams from the laser bar 34-0 is undeflected, toprovide four successive beam angles of 0, A1, A2 and A3. This isidentical to the succession of beam angles provided to the optics 42,52, in the embodiment of FIG. 10.

While the invention has been described in detail with reference topreferred embodiments, it is understood that variations andmodifications thereof may be made without departing from the true spiritand scope of the invention.

1. A dynamic surface anneal apparatus for annealing a semiconductorworkpiece, said apparatus comprising: a workpiece support for supportinga workpiece; an optical source; scanning apparatus for scanning saidoptical source and said workpiece support relative to one another alonga fast axis; wherein said optical source comprises: an array of laseremitters arranged generally in successive rows of said emitters, saidrows being transverse to said fast axis; plural collimating lensletsoverlying respective ones of said rows of emitters and providingcollimation direction along said fast axis, respective ones of saidplural collimating lenslets being aligned to provide an opticaldeflection angle corresponding to a beam deflection along said fast axisfor respective rows of emitters; and optical apparatus for focusinglight from said array of laser emitters into a narrow line beam on asurface of said workpiece along a direction transverse to said fastaxis.
 2. The apparatus of claim 1 wherein said optical apparatusproduces a first line beam on the workpiece surface from light from saidselected ones of said rows of emitters, and produces a second line beamon the workpiece surface from light from the remaining ones of said rowsof emitters, said first and second line beams being mutually paralleland transverse to said fast axis and being offset from one another alongsaid fast axis by a distance corresponding to said deflection angle. 3.The apparatus of claim 2 wherein one of said first and second line beamsis a leading beam and the other is a trailing beam.
 4. The apparatus ofclaim 2 further comprising: a fast axis beam profile controller; a firstcurrent source coupled to supply the rows of emitters covered by saidselected lenslets; and a second current source coupled to supply therows of emitters covered by remaining ones of said lenslets.
 5. Theapparatus of claim 4 wherein said fast axis beam profile controller isprogrammed to adjust the output levels of said first and second currentsources independently in accordance with a desired fast axis powerdensity profile.
 6. The apparatus of claim 5 wherein said first andsecond current sources set power densities of leading and trailing linebeams, respectively, on the workpiece surface.
 7. The apparatus of claim2 wherein the width of each line beam along said fast axis correspondsto a minimum resolvable image size.
 8. The apparatus of claim 7 whereinsaid deflection angle is such that the distance by which said first andsecond line beams are offset along said fast axis corresponds to aminimum resolvable image size.
 9. The apparatus of claim 8 wherein saiddistance of the offset by a half-maximum Gaussian width along said fastaxis of one of said line beams.
 10. The apparatus of claim 1 whereinsuccessive ones of said selected plural collimating lenslets areoriented to provide successive angles of deflection corresponding to asuccession of beam deflections along said fast axis.
 11. The apparatusof claim 10 wherein said successive ones of said selected pluralcollimating lenslets are rotated through a corresponding one of asuccession of beam deflection angles.
 12. The apparatus of claim 11wherein said optical apparatus produces a succession of line beams onthe workpiece surface from light from different ones or groups of saidrows of emitters corresponding to said succession of angles ofdeflection of said lenslets, adjacent ones of said line beams beingmutually parallel and transverse to said fast axis and being offset fromone another along said fast axis by a distance corresponding to adifference between successive ones of said deflection angles.
 13. Theapparatus of claim 12 further comprising a fast axis beam profilecontroller and a plurality of independently adjustable current sources,each of said current sources being coupled to supply rows of emittersassociated with corresponding beam deflection angles.
 14. The apparatusof claim 13 wherein said fast axis beam profile controller is programmedto adjust the output levels of said plural current sources independentlyin accordance with a desired fast axis power density profile.
 15. Theapparatus of claim 12 wherein said successive ones of said deflectionangles provides a beam-to-beam displacement along the fast axiscorresponding to a minimum resolvable spot size.
 16. The apparatus ofclaim 12 wherein said successive ones of said deflection angles providea beam-to-beam displacement along the fast axis corresponding to ahalf-maximum Gaussian width of one of said beams along the fast axis.